<!doctype html>
<!--
Copyright 2019 The Chromium Authors. All rights reserved.
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.
-->
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
  <style>
    #mainanim {
      width: 200px;
      height: 200px;
      background-color: red;
      will-change: transform;
    }
  </style>
</head>
<body>
<div id='mainanim'></div>
<hr>

<hr>
Press any key to toggle animation.

</body>
<script>

const targetMainFps =
  window.location.hash.length > 1 ? + window.location.hash.substr(1) : 0;
const skipEvery = targetMainFps > 0 ? parseInt(60 / targetMainFps) : 0;

const query = window.location.search.substr(1).split('&');
const shouldJank = query.indexOf('jank') >= 0;
const shouldExtremeJank = query.indexOf('extremejank') >= 0;
const offscreen = query.indexOf('offscreen') >= 0;
if (offscreen) {
  document.body.style.marginTop = '5000px';
}
const jankAmount = shouldExtremeJank ? 5000 : 16;

var currentValue = 0;
const maxValue = 150;
const valueChange = 5 * skipEvery;
const minValue = 0;
const attributeName = 'transform';
const setProp = (v) => 'translateX(' + v + 'px)';

var animating = false;
var incrementing = false;
var frameCount = 0;

function animateStep() {
  if (skipEvery && ++frameCount % skipEvery == 0) {
    if (incrementing) {
      currentValue += valueChange;
      if (currentValue >= maxValue)
        incrementing = false;
    } else {
      currentValue -= valueChange;
      if (currentValue <= minValue)
        incrementing = true;
    }
    if (shouldJank || shouldExtremeJank) {
      const now = new Date();
      while ((new Date() - now) < jankAmount) {}
    }
    mainanim.style[attributeName] = setProp(currentValue);
  }
  if (animating)
    requestAnimationFrame(animateStep);
}

function startAnimation() {
  animating = true;
  requestAnimationFrame(animateStep);
}

function stopAnimation() {
  animating = false;
}

function toggle() {
  if (animating) {
    stopAnimation();
  } else {
    startAnimation();
  }
}

window.onkeypress = toggle;
</script>
</html>
